1. Field of the Invention
The present invention relates to an optical head based on the use of a solid immersion lens, and an optical recording medium on which recording and reproduction are performed by using the optical head. In particular, the present invention especially relates to an optical head and an optical recording medium which improve the durable performance for sliding movement made between the optical head based on the use of a solid immersion lens and the optical recording medium. The present invention also relates to an optical recording device installed with the same optical head.
2. Description of Related Art
Recently, the optical recording device, which is an information-recording device capable of recording a large capacity of data at a high density and quickly reproducing recorded data, is used in response to the development of the information-recording device to conform to the multimedia. As for the recording medium, the optical recording device includes those based on the use of the read-only disk such as CD and laser disks in which information is stamped on the disk upon production of the disk and which are capable of only reproduction of information, those based on the use of the write-once type disk such as CD-R which are capable of recording only once, and those based on the use of the rewritable type disk in which data can be rewritten and erased any number of times by using the magneto-optical recording system or the phase-change recording system. In such an optical recording device, data is recorded and reproduced by using a light spot obtained by focusing a laser beam up to the diffraction limit by using a lens. The size d of the light spot is represented by d=.lambda./NA provided that the wavelength of the laser is .lambda., and the numerical aperture of the lens is NA ("Principle and Application of Optical Diskstorage", edited by Yoshito Tsunoda, Incorporated Association of Electronic Information and Communication Society (1995), p. 65).
In order to record information on the optical recording medium at a higher density, it is necessary to decrease the recording laser spot size so that minute pits and magnetic marks are formed. However, according to the expression described above, in order to decrease the light spot, the laser wavelength (.lambda.) may be decreased, or the numerical aperture (NA) of the lens is increased. The semiconductor laser for performing reproduction on the optical disk used at present has the wavelength which is mainly 780 to 680 nm. A laser of orange color of 650 nm, which is shorter in wavelength than the above, begins to be used, for example, for the digital versatile disk (DVD-ROM). However, the short wavelength laser, which emits light of a wavelength shorter than the wavelength of the orange laser, is still under development. There is a limit to decrease the spot size by decreasing the laser wavelength.
On the other hand, as shown in FIG. 1, the numerical aperture (NA) of the lens is represented by NA=sin .theta. provided that the focusing half angle of the lens is .theta.. The numerical aperture NA has a value smaller than 1. The lens, which is used at present, has NA of about 0.5. Even if NA=0.9, which approximates to the theoretical limit, is achieved, the laser spot size can be merely reduced to be 1/1.8 at most. On the other hand, if NA is increased, then the depth of focus of the lens becomes shallow, and a problem arises in that a complicated control system should be used to maintain the focal point on the recording plane. Therefore, it is impossible to excessively increase NA. In the case of an ordinary optical recording device, a lens approximately having NA=0.6 is used at the maximum.
A method for effectively increasing NA of the lens has been suggested, in which a solid immersion lens is used in order to decrease the spot size of the laser beam (Nikkei Electronics, No. 686, pp. 13-14, 1997.4.7). As shown in FIG. 2A, when a hemispherical solid immersion lens is used, and the laser beam is allowed to come perpendicularly into the lens surface, then the equivalent NA of the optical system is represented by n.times.NA provided that the refractive index of the solid immersion lens is n. As shown in FIG. 2B, when a super spherical solid immersion lens is used, and the laser beam is allowed to come so that the focal point is formed on the bottom surface of the super spherical lens, then the equivalent NA is represented by n2.times.NA. When the solid immersion lens is made of glass, the refractive index of glass is about 1.8. Therefore, the spot size can be decreased to be 1/1.8 when the hemispherical solid immersion lens is used, and the spot size can be decreased to be 1/3.2 when the super spherical solid immersion lens is used, respectively as compared with the case in which an ordinary objective lens is used.
When the solid immersion lens is used, the near field light, which leaks out from the solid immersion lens, is used for recording and reproduction in this technique. Therefore, it is necessary that the spacing distance between the solid immersion lens and the recording film is about 1/4 of the wavelength of the laser at most. The value corresponding to thereto is 170 nm when a red laser having a wavelength of 680 nm is used. Therefore, the spacing distance is extremely smaller than the spacing distance of several mm between the optical head and the optical recording medium used in an ordinary optical recording device. Accordingly, when the solid immersion lens and the near field light are used in combination, it is necessary to use a floating type slider which is used for the magnetic head for the fixed type magnetic disk (hard disk).
FIG. 3 shows an example of the structure of the optical head for the magneto-optical recording medium based on the use of such a floating type slider. The optical head comprises an objective lens 71, a solid immersion lens 100, and a recording magnetic field-generating coil 104 incorporated into a floating type slider 102. In the case of an ordinary magneto-optical recording device, the light is radiated onto the recording layer through a transparent substrate of the magneto-optical recording medium. However, in the case of the optical head based on the use of the solid immersion lens, the spacing distance between the solid immersion lens and the recording layer is restricted as described above. Therefore, the magneto-optical recording medium adopts a structure in which a reflective layer, a first dielectric layer, a magneto-optical recording layer, and a second dielectric layer are stacked in this order on a substrate. It is necessary for the magneto-optical recording medium to be irradiated with the recording and reproducing light beam from the side of the second dielectric layer. The floating type slider adopts the CSS (contact start and stop) system in the same manner as the magnetic head. Therefore, it is necessary that the surface of the magneto-optical recording medium has a protective layer and a lubricating layer capable of withstanding CSS.
In the case of the ordinary optical disk, in order to protect the recording film, a protective film of several pm to several tens .mu.m is formed on the side of the recording film surface opposite to the substrate by applying ultraviolet-curable resin or Si resin curable in the atmospheric air. However, in the case of the system based on the use of the solid immersion lens and the near field light, it is impossible to form the protective film made of resin on the second dielectric layer, because the protective film made of resin is thicker than the leakage distance of the near field light. Therefore, in this system, the recording and reproducing optical head is moved at a position separated by about 100 nm from the second dielectric film as the uppermost layer, in the same manner as in the fixed type magnetic disk device. For this reason, if the optical head varies its floating posture during the movement, then the optical head contacts with the second dielectric film, and its surface is scratched in some cases.
The second dielectric film, which is disposed at the uppermost layer of the magneto-optical recording medium, is formed of a hard material such as silicon nitride, silicon oxide, aluminum nitride, and silicon carbide. The film thickness thereof is 50 to 100 nm which is two to five times thicker than that used in the magnetic disk. The floating height of the recording and reproducing optical head can be 100 to 150 nm which is about two to three times higher than that used in the magnetic disk, when the solid immersion lens and the near field light are used. Therefore, the scratch, which is formed on the surface of the dielectric film due to irregular sliding movement caused by variation of the posture of the optical head, does not become so deep to arrive at the recording film. In many cases, the scratch is formed in the traveling direction of the optical head, as a grazed stripe-shaped scratch having a width of about several .mu.m to several hundreds .mu.m and a depth of about several tens nm. In the case of the optical disk such as an ordinary magneto-optical disk, the laser spot size is about 1 mm at the minimum on the surface of the substrate, because the reproduction is performed through the transparent substrate. Therefore, the scratch of about several .mu.m to several tens .mu.m formed on the substrate surface scarcely causes problems concerning recording and reproduction. However, the system, which is based on the use of the solid immersion lens and the near field light, utilizes the leakage of the laser beam having been focused up to the diffraction limit. Therefore, a problem arises in that the scratch, which merely has a width of several .mu.m formed on the surface of the dielectric film, tends to cause any reproduction error due to variation of the amount of reflected light, i.e., the amount of reproducing light, caused by the interference at the edge of the scratch.
The present invention has been made in order to dissolve the drawbacks of the conventional technique described above, an object of which is to provide an optical recording medium in which the scratch is hardly formed on the recording medium surface due to collision between the optical head and the recording medium, and even if any scratch is formed on the medium surface, the scratch is in a degree not to cause any error during reproduction.
Another object of the present invention is to provide an optical head and a recording and reproducing device for an optical recording medium based on the use of the optical on the medium surface, the scratch is in a degree not to cause any error during reproduction.